The critical early-voting state of South Carolina is emerging as a crucial front in the rivalry between Sen. Marco Rubio (R-Fla.) and former Florida governor Jeb Bush. Expectations for Rubio are soaring, Republicans say, putting pressure on Bush in a state his brother and father won — a state which has earned a reputation as a firewall protecting front-runners from insurgent underdogs.

Rubio, an underdog for the nomination who faces opponents with longer political resumes and deeper roots in the GOP, is in some ways an unlikely bet for South Carolina. As of now, he is averaging under 5 percent support there in recent polling.

But for years, he has been laying a foundation, often informally, in the vital early nominating state where he will officially campaign for president for the first time on Saturday.

The operatives trying to get Rubio elected also illustrate how hard he intends to compete in the state. Rubio’s team is being led by a small army of seasoned South Carolina operatives. Bush is relying in South Carolina on two strategists who have spent most of their professional careers outside the state.

Beyond staffing, Rubio’s political platform presents Bush with a challenge. His attempt to appeal to both ends of the GOP spectrum — he was elected to the Senate as a conservative rebel but has since also endeared himself to the establishment wing — and his aggressive national security posture is ideal for South Carolina, Republicans say.

“He should do very well in South Carolina because they like that hybrid tea party-establishment person who is also a defense hawk,” said Ford O’Connell, who worked on the 2008 presidential campaign of Sen. John McCain (R-Ariz.).